The present invention relates to image capture devices, such as x-ray sensors, and more particularly to a digital (pixelized) scintillation layer.
Image capture devices of the type to which the present invention pertains are typically designed to capture relatively large images employing a radiation source outside the visible light spectrum, for example those employing an x-ray source. Due to the large image area size, for example greater than several square inches, image capture device in this class will generally include an amorphous silicon (a-Si:H) sensor array. This array includes a plurality of pixels, each containing at least a photodiode and a transistor connected to data and scan lines. Other devices of the type to which the present invention pertains include CCD image sensors and CMOS image sensors, both of which being typically smaller than a-Si:H arrays. Diode-addressing-logic rather than transistor logic may also be employed to read out the a-Si:H array.
Radiation outside of the visible light spectrum cannot be directly detected efficiently by an a-Si:H sensor. Rather, the source radiation must be converted into visible light prior to its detection by the sensor array. This is accomplished by a scintillation layer, often disposed immediately adjacent to the sensor array. A scintillator, or scintillation layer, is a layer of material that emits optical photons in response to ionizing radiation. Optical photons are photons with energies corresponding to wavelengths between 3,000 and 8,000 angstroms. Thus, the scintillation layer converts source radiation energy, such as x-ray, into visible light energy, which may then be detected by the sensor array. Since the effect of a scintillation layer is typically to convert relatively few, high energy source photons into relatively many, low energy optical photons, such layers are also known as photomultiplier layers. When a scintillation layer is combined with a support layer (such as polyester film), the combination is known as screen or an x-ray intensifying screen.
Examples of scintillation layer material for this application include GdO.sub.2 S.sub.2, Csl, Csl:TI, BaSO.sub.4, MgSO.sub.4, SrSO.sub.4, Na.sub.2 SO.sub.4, CaSO.sub.4, BeO, LiF, CaF.sub.2, etc. A more inclusive list of such materials is presented in U.S. Pat. No. 5,418,377, which is incorporated herein by reference. Commercial scintillation layers may contain one or more of these materials, and screens incorporating such mixtures are sold under the trademarks Trimax, from 3M Corp., Cronex, from Dupont Corp., and Lanex, from Kodak Corp.
Resolution is a critical criteria for any image capture device. In the case of devices of the type described above, a number of factors determine device resolution. However, the focus for the purposes of this description is on the effects the scintillation layer material and structure have on resolution. If a continuous, homogeneous scintillation layer is used, for example in devices in which one of the aforementioned commercial intensifying screens is applied directly over a sensor array, scattering and multiple reflections within the intensifying screen distribute the light energy from the point of generation. This results in a distribution of light over several or more discrete sensors, or pixels, and is referred to as an increase in the line spread function (LSF), and a degradation of the modulation transfer function (MTF). For a scintillation layer having an attenuation constant .mu., and thickness d, the MTF at spatial frequency .rho. is the Fourier transform of LTF, and is given by [reference Albert Macovski, "Medical Imaging Systems," Prentice Hall, 1983, pp. 66] ##EQU1##
FIG. 1 is an illustration of the effects of this distribution, showing those relevant portions of an image capture device 2, although not to scale. Device 2 includes a sensor array 12, having numerous pixels identified as 14.sub.n-3, 14.sub.n-2, 14.sub.n-1, 14.sub.n, 14.sub.n+1, 14.sub.n+2, 14.sub.n+3 etc., and a continuous, homogeneous scintillation layer 22 disposed over array 12. A radiation source 24 emits radiation energy e, which may be partly or completely absorbed, scattered or transmitted by subject 26. Transmitted radiation energy is incident upon scintillation layer 22. When a photon from radiation source 24 excites material in scintillation layer 22, its energy is converted into optical photons, the extent of which may be detected by one or more of pixels 14.sub.n etc. The detection by pixels 14.sub.n etc. is read out and controlled by circuitry 16, which may, for example, cause the image to be displayed on a monitor 18 or the like (the details of which being beyond the scope of this invention).
Importantly, when the optical photons spread out and are scattered within scintillation layer 22 they are detected by more than one of pixels 14.sub.n etc. This effect is illustrated by the width w of the plot 4 of intensity versus position for a line of source photons striking scintillation layer material, referred to as the Line Spread Function, shown in FIG. 1. It will be appreciated that the narrower the width of such a plot, the narrower the distribution of the optical photons within the scintillation layer 22, and hence the better the resolution performance (image clarity and accuracy) of the device, since (a) the location of the point of incidence of the source radiation can be more accurately determined, and (b) the signal loss is reduced and a more accurate sensing of the energy of the optical photons can be made.
Table 1 list results of measured performance of various scintillation layers, and illustrates the tradeoff between resolution and efficiency, where .eta.=1-e.sup.-.mu.d is the fraction of incident x-ray photons that are absorbed by the scintillation material, and .rho..sub.10% is the value of .rho. such that MTF(.rho.)/MTF(0)=10%. A known benefit of solid state image capture devices is the ability to obtain an image with a lower source radiation dosage than typical film image capture devices (i.e., x-ray). So, efficiency is a critical parameter for image capture devices, since a decrease in efficiency results in an increase of the required dosage of source radiation needed to obtain an image. The various scintillation layers in Table 1 are manufactured by Kodak, contain GdO.sub.2 S.sub.2, and are sold under the trademark Lanex.
TABLE 1 Screen Film .eta. (55 KeV) .rho..sub.10% (90 KeV) [mm.sup.-1 ] Fast TMG .75 3.0 Regular TMG .58 3.5 Medium TMG .41 4.3 Fine TMG .18 8.8
There are several ways known to counteract the spreading out of the optical photons within scintillation layer 22. The first is to reduce the thickness d of the layer. This reduces the distance the optical photons may travel in the scintillation layer. However, the thinner the scintillation layer, the lower its conversion efficiency, since there is less scintillating material with which a source photon may collide. This thickness/resolution tradeoff is well known in the art. See, e.g., U.S. Pat. No. 4,069,355.
Another approach known in the art is to employ thallium doped cesium iodide (Csl:TI) as a scintillation layer. Csl:TI is deposited as a film in thickness up to 400 .mu.m by a high temperature process such as vacuum sputtering. There is generally a relatively large mismatch between the thermal expansion coefficient of the substrate and of Csl:TI. As the two bodies cool, the stresses resulting from the mismatch cause micro cracks to form in the Csl:TI structure. These cracks run perpendicular to the plane of the deposited film, and are generally spaced apart by between 10 and 20 .mu.m. The cracks form boundaries through which the optical photons do not pass. Thus, confinement structures are formed in the scintillation layer, and the Csl:TI layer may be made relatively thick without thereby degrading resolution. This type of structure, and indeed any in which the scintillation material confines the dispersion of optical photons in a direction in the plane of the scintillation layer, is referred to herein as a pixelized scintillator.
This approach has several disadvantages. First, Csl is a toxic material. And in fact, TI is a very toxic material. Thus, using such material presents environmental health and safety concerns, as well as special permitting requirements for facilities handling this material. Second, films of Csl:TI are very fragile, and special handling procedures must be employed during manufacture of the films and devices employing the films. Third, Csl:TI is hygroscopic. Water attracted by the film negatively effects luminescence. Thus, additional processing, use of desiccants, etc. are required.
An alternative to the basic Csl:TI application is the creation of physically isolated, columnar structures of scintillation material. There are numerous ways to accomplish this. For example, U.S. Pat. No. 3,041,456 teaches forming a layer of scintillation material, dicing said layer, and reassembling same such that the joints between adjacent die present an optical boundary. However, die cutting requires substantial handling and introduces manufacturing inconsistencies. Furthermore, resolution is limited due to the practical limit on the size of each die.
U.S. Pat. No. 3,936,645 teaches creating laser-cut slots between regions of scintillation material, and filling said slots with optically opaque material. U.S. Pat. No. 5,418,377 teaches laser ablation of a continuous scintillation layer to form discrete scintillation material regions. These laser processing techniques cannot produce acceptable resolution, however, as the limit of control of the laser is too large to obtain the desired region-to-region spacing. Furthermore, the ablation process produces debris which affects performance of the scintillation material and introduces region-to-region variation in response. Finally, the process is relatively complex, difficult to control, and expensive.
U.S. Pat. No. 4,069,355 teaches forming a pixelized scintillation layer by depositing Csl onto pads formed in or on a substrate. The Csl selectively grows on the pads to form columnar scintillation structures. U.S. Pat. No. 5,368,882 teaches forming scintillation material on mesas formed with sloped walls, again so that the scintillation material selectively grows in the form of columns. These alternatives also present significant problems. For example, the process of forming the pads is relatively complex, with numerous steps, introducing complexity and/or yield issues. Also, it is difficult to form such layers over regions larger than a few square inches. Lastly, because it uses Csl, it suffers from the disadvantages previously mentioned regarding that material.
U.S. Pat. No. 5,171,996, teaches forming depressions in etchable substrate material, such as glass, plastic, a ceramic, a thin metal layer such as Al or Ti, or crystalline or amorphous silicon or germanium. The surface of the etched substrate is then covered with scintillation material by vacuum deposition. Properties of the evaporation are used to confine the deposited material to columns located in the depressions etched 5-20 .mu.m into the substrate. The columns then extend out of the depressions by 300-1000 .mu.m. The depth of 5-20 .mu.m of the depression is carefully controlled as required by the deposition process taught by the reference to allow the scintillation material to be selectively deposited therein. Should, for example, the depression depth exceed the specified 20 .mu.m, the process results in the deposition of the scintillation material not only in the depressions, but also on the ridges (element 16 in the reference) between the depressions. This reduces the effective separation between columns of scintillation material (element 19 in the reference), resulting in the problems associated with continuous films of scintillation material, such as loss of resolution, etc., since the reference relies on the air or vacuum gaps (elements 20 in the reference) to isolate the columns.
Accordingly, there is a need in the art for an improved pixelized scintillation layer providing high resolution, high conversion efficiency, environmental safety, ruggedness, and an improved method for making same.